(1) Field of the Invention
The present invention relates to micro mirror technology using MEMS (Micro Electro Mechanical Systems). The invention relates particularly to micro mirror units suitable for use in large-scale channel switching (crossconnect) for wavelength division multiplex signals with multiple wavelengths (channels), and to the manufacturing process for the micro mirror units. Further, the invention also relates to an optical switch with such micro mirror units employed therein.
(2) Description of Related Art
With recent speeding up of optical signals transmitted over backborn networks, it is necessary for optical switches such as optical crossconnect devices to support very high-speed optical signals as fast as 10 Gbps (Gigabits per second). In addition, with increases in the number of channels transmitted in WDM (Wavelength Division Multiplex) systems, the scale of switching has been becoming extremely large.
Against this backdrop, optical switches employing micro tilt mirror arrays with MEMS have been developed for use as large-scale optical switches. Such example optical switches are proposed in the article, “D. T. Neilson, et al., ‘Fully provisioned 112×112 micro-mechanical optical crossconnect with 35.8T b/s demonstrated capacity,’ Optical Fiber Communications Conference (OFC 2000), Postdeadline paper PD-12, March 2000”, and in International Patent Publication No. WO 00/20899. Further, MEMS micro tilt mirrors disclosed in U.S. Pat. No. 6,044,705 are well known.
Now, a description will be made of MEMS micro tilt mirrors.
FIG. 9A schematically shows a top view of a MEMS micro tilt mirror (unit); FIG. 9B shows a side view on arrow A of FIG. 9A. As shown in FIG. 9A and FIG. 9B, the micro tilt mirror includes a bottom substrate (hereinafter will be simply called the “substrate”) 3 with the shape of a letter U (with walls 31 and 32) in the side view on arrow A, on which substrate 3 there are arranged a micro mirror (hereinafter will be simply called the “mirror”) 1, torsion bars 2, which are integrated with the mirror 1, and electrodes 5. The torsion bars 2 are rotatably supported by the walls 31 and 32 of the substrate 3.
As shown in FIG. 9B, this arrangement provides a space between the mirror 1 and the substrate 3 with a height equal to the height of the walls 31 and 32. By applying a specific voltage to the electrodes 5, an electromagnetic force is generated at the electrodes 5 so as to turn the mirror 1 about the torsion bar 2. A micro tilt mirror with such a construction is manufactured as follows. (1) Electrodes 5 are formed on the substrate 3 by etching or the like. (2) After a sacrifice layer 4 of silicon oxide (SiO2) resin or the like is formed on the electrodes 5 and the substrate 3, (3) a mirror 1 and torsion bars 2 are formed on the sacrifice layer 4 by etching. (4) The sacrifice layer 4 is finally removed by wet etching with use of a predetermined removal agent.
In large-scale optical switches employing such micro tilt mirrors, the greater the number of channels to be subjected to switching is, the greater the required turn angles (tilt angles) of mirrors 1. Here, the tilt angle of each mirror 1 can be enlarged by increasing the height (distance) between the substrate 3 and the mirror 1. However, as shown in FIG. 10, since the distance between the mirror 1 and the substrate 3 depends on the thickness 7 of the sacrifice layer 4, which is formed between the electrode 5 and mirror 1 during the manufacturing process, the thickness 7 of the sacrifice layer 4 needs to be increased so as to enlarge the tilt angle of the mirror 1.
Hence, in previous arts, the thickness 7 of the sacrifice layer 4 is increased to realize an increased height between the substrate 3 and the mirror 1. However, the increased thickness 7 makes it difficult to completely remove the sacrifice layer 4 by a wet etching method, so that some part (remaining sacrifice layer) 6 of the sacrifice layer 4 is left unremoved as shown in FIG. 9A and FIG. 9B. This means that the mirror 1, the substrate 3, and the electrodes 5, are fixed to one another, thereby making it impossible for the mirror 1 to operate. In such cases, the thickness of the sacrifice layer 4 cannot be increased, mirror tilt angles are resultantly limited, and yields are deteriorated.
With the foregoing problems in view, one object of the present invention is to provide micro mirror units with enlarged mirror tilt angles, without increasing the thickness of the sacrifice layer. Another object of the invention is to provide a method for manufacturing the micro mirror units. A further object of the invention is to provide optical switches with the micro mirror units employed therein.